Front Axle - 248FBI
DESCRIPTIONThe housing for the 248 Front Beam-design Iron (FBI) axle consists of an iron center casting with tubes on each side. The tubes are pressed into and welded to the differential housing.
The integral type housing, hypoid gear design has the centerline of the pinion set below the centerline of the ring gear.
The axle has a vent used to relieve internal pressure caused by lubricant vaporization and internal expansion.
The axles are equipped with semi-floating axle shafts, meaning that loads are supported by the hub bearings. The axle shafts are retained by nuts at the hub bearings. The hub bearings are bolted to the steering knuckle at the outboard end of the axle tube yoke. The hub bearings are serviced as an assembly.
The axles are equipped with ABS brake sensors. The sensors are attached to the knuckle assemblies and the tone rings are pressed onto the axle shaft. Use care when removing axle shafts as NOT to damage the tone wheel or the sensor.
The stamped steel cover provides a means for inspection and servicing the differential.
The 248 FBI axle have the assembly part number and gear ratio listed on a tag. The tag is attached to the housing cover by one of the cover bolts. Build date identification codes are stamped on the cover side of a axle tube.
The differential case is a one-piece design. The differential pinion mate shaft is retained with a roll pin. Differential bearing preload and ring gear backlash is adjusted by the use of shims. The shims are located between the differential bearing cones and case. Pinion bearing preload is set and maintained by the use of a collapsible spacer.
The axle differential covers can be used for identification of the axle (Fig. 1). A tag is also attached to the cover.
OPERATION
The axle receives power from the transfer case through the front propeller shaft. The front propeller shaft is connected to the pinion gear which rotates the differential through the gear mesh with the ring gear bolted to the differential case. The engine power is transmitted to the axle shafts through the pinion mate and side gears. The side gears are splined to the axle shafts.
During straight-ahead driving, the differential pinion gears do not rotate on the pinion mate shaft. This occurs because input torque applied to the gears is divided and distributed equally between the two side gears. As a result, the pinion gears revolve with the pinion mate shaft but do not rotate around it (Fig. 2).
When turning corners, the outside wheel must travel a greater distance than the inside wheel to complete a turn. The difference must be compensated for to prevent the tires from scuffing and skidding through turns. To accomplish this, the differential allows the axle shafts to turn at unequal speeds (Fig. 3). In this instance, the input torque applied to the pinion gears is not divided equally. The pinion gears now rotate around the pinion mate shaft in opposite directions. This allows the side gear and axle shaft attached to the outside wheel to rotate at a faster speed.